1. Field of the Invention
This invention generally relates to a method and a circuit for driving a liquid crystal display, and more particularly to a Vcom inversion drive method and a circuit for driving a liquid crystal display.
2. Description of Related Art
There are several conventional drive methods for driving active liquid crystal displays (LCDs), such as a frame inversion method, a column inversion method, a row inversion method, and a dot inversion method. Anyone of the above methods can be chosen based on image quality, power consumption, and drive complexity. The frame inversion method is the simplest of the above methods, but provides a worst image quality compared to the other methods. Hence, the frame inversion method is rarely adopted. The dot inversion method can provide a best image quality, but requires a higher power and a complex driving circuitry. Hence, the column inversion method and the row inversion method are the most common methods adopted by users when the image quality is not critical.
FIG. 1 is an equivalent circuit of a conventional pixel driving circuit for driving an active LCD. A scan signal VS 102 is sent to a gate of a transistor 110 to turn the transistor 110 on/off. A data signal VD 104 is sent to a source of the transistor 110. When the transistor 110 is on, the data signal 104 will go through the transistor 110 to obtain an internal voltage 112 (Vlc). The internal voltage 112 then is stored in a storage capacitor 130 (Cst) and a liquid crystal pixel having a capacitance (schematically represented as a liquid crystal capacitor 120 (Clc)). The liquid crystal (schematically represented as the liquid crystal capacitor 120) will be driven based on a voltage difference between the internal voltage 112 and the voltage (Vcom) of a DC signal 106. When the transistor 110 is off, the storage capacitor 130 will provide a required voltage level for driving the liquid crystal capacitor 120.
FIG. 2 shows a time sequences for signals illustrated in FIG. 1. Referring to FIG. 2, a DC signal (Vcom) 206 is a reference voltage. When a scan signal (VS) 202 is at a high voltage level, a data signal (VD) 204 enters into a pixel and charges the storage capacitor 130 and the liquid crystal capacitor 120. Hence, an internal voltage 212 can maintain a stable level to provide a voltage difference 210 for the liquid crystal capacitor 120.
The aforementioned inversion driving method causes higher power consumption because the inversion has to be made each time after the data signal enters the pixel, which requires higher voltage amplitude and higher inversion frequency.
FIG. 3 is an equivalent circuit of a conventional pixel driving circuit for driving active LCD by using Vcom inversion (a.k.a. common toggle) drive method. A scan signal 302 is sent to a gate of a transistor 310 to turn the transistor 310 on/off. A data signal 304 is sent to a source of the transistor 310. When the transistor 310 is on, the data signal 304 will go through the transistor 310 to obtain an internal voltage 312. The internal voltage 312 then is stored in a storage capacitor 330 and a liquid crystal capacitor 320. One end of the storage capacitor 330 and one end of the liquid crystal capacitor 320 are coupled to a drain of the transistor 310, the other end (common electrode) of the storage capacitor 330 and the other end of the liquid crystal capacitor 320 are connected to an AC signal 306. The liquid crystal (i.e., having the liquid crystal capacitor 320) will be driven based on a voltage difference between the internal voltage 312 and the AC signal 306. When the transistor 310 is off, the storage capacitor 330 will provide the required voltage level for driving the liquid crystal capacitor 320.
FIG. 4 shows a time sequences for signals illustrated in FIG. 3. Referring to FIG. 4, an AC signal (Vcom) 406 is a reference voltage. When a scan signal (VS) 402 is at a high voltage level, a data signal (VD) 404 enters into a pixel and charges the storage capacitor 330 and the liquid crystal capacitor 320. Hence, an internal voltage 412 can maintain a stable level to provide a voltage difference 410 for the liquid crystal capacitor 320.
The Vcom inversion driving method can reduce the amplitude of the data signal 404. Hence, the required power consumption can reduce. However, because a common electrode of the storage capacitor 330 and that of the liquid crystal capacitor 320 are coupled to the same voltage level, if the storage capacitor 330 is an asymmetric capacitor with polarity, the Vcom inversion driving method cannot be used.
As the manufacturing process advances, the size of the thin film transistor (TFT) is getting smaller and smaller. Hence, the alignment precision is more critical. The traditional optical alignment would not be able to meet the precision requirement. Although the self-aligned manufacturing process can meet the precision requirement and improve the TFT performance, it also results in an asymmetric storage capacitor between the gate electrode and the polysilicon electrode. Hence, the conventional Vcom inversion drive method cannot be used to reduce the power consumption.